Many typical RF communications systems have RF receivers that need to receive a desired RF signal on a specific RF channel, which is a desired RF channel that has a desired bandwidth and a desired RF center frequency. One function of the RF receiver is to reject any RF signals at frequencies other than those within the desired bandwidth of the desired RF channel; therefore, numerous filtering and signal rejection techniques have been developed to achieve this function. One such technique is called super-heterodyning, in which received RF signals are filtered and then mixed with a local oscillator signal to down convert the filtered RF signals into lower frequency signals, which are known as intermediate frequency (IF) signals. The mixing down converts a desired RF signal into a desired IF signal having a desired IF center frequency. Generally, it is easier to filter out unwanted signals at IF frequencies than at higher RF frequencies; however, mixers have a characteristic that produces image signals in addition to desired signals. Image signals may be removed by RF filtering, IF filtering, or both. Bandpass filtering may be used to pass desired signals and remove interfering signals. Notch filtering may be used to target and reduce interfering signals at specific frequencies. The quality factor of a bandpass filter is a measure of the effectiveness of the filter at removing interfering signals at nearby frequencies, and is the bandwidth of the filter divided by the center frequency of the filter.
Some RF communications protocols include as many channels as possible in a given bandwidth; therefore, channel spacing may be tight. To reduce interference from adjacent and alternate channels, the bandwidth of the filtering needs to be as narrow as possible, which may be difficult at RF frequencies; however, filtering out interfering signals before down conversion is preferable to reduce image problems and reduce the magnitude of interfering signals that must be handled by down conversion and downstream circuitry. Thus, there is a need for a narrow bandwidth RF bandpass filter having a high quality factor.